In the quest were you must fully level your Sunsinger subclass and talk to Ikora after(I already had it maxed out before the taken king had dropped) it told me I still hadnt so it seemed like I couldnt finish the quest because of a bug. You may want to look into this Bungie. In the mean time if you are having this same issue, I got around it by simply abandoning the mission and picking it back up at the "abandoned missions" kiosk in the Hall of Guardians. This seemed to fix it for me.